1. Field of the Invention
The present invention relates generally to phase-locked loop structures.
2. Description of the Related Art
Phase-locked loop structures are used in a wide variety of modern electronic systems (e.g., signal-conditioning systems, signal-generating systems and communication systems) that require stable signals whose frequencies can be easily selected (i.e., synthesized) and whose close-in spectrum approximates that of a stable reference oscillator (e.g., a crystal oscillator).
The phase-locked loop of these structures is generally completed around a voltage-controlled oscillator that generates an oscillator signal whose oscillator frequency varies in response to a control voltage. The frequency of this oscillator can be substantially greater than that of the reference oscillator and yet its close-in jitter (signal instability) will be controlled by the feedback loop to be a function of the low jitter of the stable reference oscillator. Because the jitter outside the bandwidth of the feedback loop remains that of the oscillator itself and because jitter generally reduces as oscillator frequency increases, the loop structure is often configured to facilitate higher oscillator frequencies.
The loop-reduced jitter will, however, be degraded by increases of the oscillator's gain (the ratio of oscillator frequency to control voltage). The oscillator gain itself is a function of operating conditions (e.g., temperature and voltage supply differences) and, in production, will also vary between upper and lower process corners that are determined by a number of process variables.
Accordingly, conventional phase-locked loop structures generally compromise signal stability because they increase the oscillator gain sufficiently to insure that, under all operating and process variations, the phase-locked loop can drive the oscillator's frequency to where it is phase locked to the reference signal. That is, this locking insurance is gained at the cost of degraded stability of the loop's output signal.
In order to maintain a desired loop bandwidth, the increased oscillator gain is generally offset by altering loop compensation elements. In particular, it is typically offset by increasing loop compensation capacitors which subtracts from circuit area which is always a limited resource in integrated-circuit realizations of phase-locked loops.